Piezoelectric thin film resonator, filter, communication module and communication device

ABSTRACT

A piezoelectric thin film resonator includes a substrate, a lower electrode provided on the substrate, a piezoelectric film provided on the lower electrode, and an upper electrode provided on the piezoelectric film. At least a portion of the upper electrode and that of the lower electrode oppose each other through the piezoelectric film, and at least a portion of the periphery of the upper electrode is reversely tapered.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of a pending application, U.S. Ser. No.12/627,412 filed on Nov. 30, 2009, which is hereby incorporated byreference in its entirety. The parent application is based upon andclaims the benefit of priority of the prior Japanese Patent ApplicationNo. 2009-68019 filed on Mar. 19, 2009, the entire contents of which isincorporated herein by reference.

FIELD

The disclosure of the present application relates to piezoelectric thinfilm resonators. The disclosure of the present application relates tofilters, communication modules and communication devices including thepiezoelectric thin film resonators.

BACKGROUND

In recent years, SAW (surface acoustic wave) filters and BAW (bulkacoustic wave) filters have been used widely for RF filters for mobilecommunications such as for mobile phones. BAW filters includepiezoelectric thin film resonators. There are two types of piezoelectricthin film resonators: FBAR (Film Bulk Acoustic Resonator) and SMR(Solidly Mounted Resonator). An FBAR has a structure in which an upperelectrode, a piezoelectric film and a lower electrode are provided on asubstrate as main components, and an air gap is formed under the lowerelectrode at a portion where the upper electrode and the lower electrodeoppose each other. Here, the air gap is formed by wet etching asacrificial layer provided on the surface of or inside the substrate, orwet etching or dry etching the substrate from the backside. On the otherhand, an SMR has a structure in which, instead of the air gap, alaminate with a film thickness of λ/4 (λ: wavelength of acoustic wave)formed by alternately laminating films having a high acoustic impedanceand films having a low acoustic impedance is provided, and the laminateis utilized as an acoustic reflection film.

In particular, filters and branching filters using BAWs have beenreceiving attention due to their higher Q-value even at high frequenciesand smaller losses than those using conventional SAWs. However, asdemands for lower power consumption in the field of mobilecommunications have become vigorous in recent years, the filters and thebranching filters using BAWs are demanded for a further reduction inlosses. Because of these reasons, low-loss piezoelectric thin filmresonators have been actively developed.

One of the factors that contributes to losses in such filters using BAWsis that acoustic waves leak to the outside (hereinafter referred to as anon-resonance portion) of an area where the upper electrode and thelower electrode oppose each other (hereinafter referred to as aresonance portion), in other words, acoustic waves leak into an areawhere they are less likely to be reconverted into electric signals, andthereby causing losses. Herein, this phenomenon will be referred to as a“lateral leakage”. The lateral leakage is caused by the magnituderelationship in acoustic velocity between the resonance portion and thenon-resonance portion. The magnitude relationship in acoustic velocitythat suppresses the lateral leakage is determined by the Poisson's ratioof a piezoelectric material to be used. The acoustic velocity in theresonance portion becomes lower than that in the non-resonance portionwhen the Poisson's ratio is 1/3 or more, and the acoustic velocity inthe resonance portion becomes higher than that in the non-resonanceportion when the Poisson's ratio is 1/3 or less.

Here, in a case where a piezoelectric film is formed using a materialwhose Poisson's ratio is 1/3 or more, the acoustic velocity in theresonance portion becomes lower than that in the periphery when anappropriate amount of mass is added to the resonance portion. Thus, thelateral leakage can be suppressed with relative ease. In contrast, in acase where a piezoelectric film is formed using a material whosePoisson's ratio is 1/3 or less, the acoustic velocity relationship thatsuppresses the lateral leakage becomes opposite. Thus, it is difficultto suppress the lateral leakage. In currently-practical filters usingpiezoelectric thin film resonators, AlN, whose Poisson's ratio is 1/3 orless, is used for the piezoelectric films. Thus, it is difficult tosuppress the lateral leakage, and as a result the losses increase.

As a way to solve the lateral leakage of acoustic waves, JapaneseLaid-open Patent Application No. 2007-300430 discloses a resonator inwhich the piezoelectric film in the resonance portion is subjected topatterning and at least a portion of the periphery of the patternedpiezoelectric film is provided inwardly than an area where the upperelectrode and the lower electrode oppose each other. By using theresonator disclosed in Japanese Laid-open Patent Application No.2007-300430, the lateral leakage of acoustic waves can be suppressed ina highly effective manner.

The Q-value of the resonator disclosed in the patent document can beincreased by increasing the over-etching amount of the piezoelectricfilm to further increase the length of a hood-like end portion. When thelength of the hood-like end portion is increased, the mechanicalstrength of the end portion is difficult to maintain. Accordingly, withthe resonator disclosed in the patent document, it is difficult toincrease the Q-value while maintaining the mechanical strength of theend portion.

SUMMARY

According to an aspect of the invention, a piezoelectric thin filmresonator includes: a substrate; a lower electrode provided on thesubstrate; a piezoelectric film provided on the lower substrate; and anupper electrode provided on the piezoelectric film. At least a portionof the upper electrode and that of the lower electrode oppose each otherthrough the piezoelectric film, and at least a portion of a periphery ofthe upper electrode has a reversely tapered shape.

Additional objects and advantages of the invention (embodiment) will beset forth in part in the description which follows, and in part will beobvious from the description, or may be learned by practice of theinvention. The object and advantages of the invention will be realizedand attained by means of the elements and combinations particularlypointed out in the appended claims. It is to be understood that both theforegoing general description and the following detailed description areexemplary and explanatory only and are not restrictive of the invention,as claimed.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A is a plan view illustrating a conventional piezoelectric thinfilm resonator.

FIG. 1B is a cross-sectional view of the portion Z-Z in FIG. 1A.

FIG. 2A is a plan view illustrating a piezoelectric thin film resonatoraccording to one embodiment.

FIG. 2B is a cross-sectional view of the portion Z-Z in FIG. 2A.

FIG. 3A is a plan view illustrating a piezoelectric thin film resonatorwith an elliptic resonance portion.

FIG. 3B is a cross-sectional view of the portion Z-Z in FIG. 3A.

FIG. 4A is a plan view illustrating a piezoelectric thin film resonatorwith a pentagonal resonance portion.

FIG. 4B is a cross-sectional view of the portion Z-Z in FIG. 4A.

FIGS. 5A to 5L are cross-sectional views each illustrating a productionprocess of a piezoelectric thin film resonator according to Example 1.

FIGS. 6A to 6K are cross-sectional views each illustrating a productionprocess of a piezoelectric thin film resonator according to Example 2.

FIG. 7 is a cross-sectional view illustrating a piezoelectric thin filmresonator in which an air gap is formed in the substrate.

FIG. 8A is a cross-sectional view illustrating a model of apiezoelectric thin film resonator.

FIG. 8B is a characteristic diagram illustrating the relationshipbetween the angle of an end portion of an upper electrode and ananti-resonance Q-value.

FIG. 9 is a plan view illustrating a filter.

FIG. 10 is a block diagram illustrating a communication module.

FIG. 11 is a block diagram illustrating a communication device.

DESCRIPTION OF EMBODIMENT(S)

According to an aspect of the invention, the periphery of the upperelectrode is placed so as to coincide with or to be in the vicinity ofat least a portion of a periphery of the piezoelectric film. Byconfiguring the piezoelectric thin film resonator in this way thelateral leakage of acoustic waves can be reduced while maintaining themechanical strength of the periphery of the upper electrode and theQ-value can be increased.

According to an aspect of the invention, an area where the upperelectrode and the lower electrode oppose each other has an ellipticshape. By configuring the piezoelectric thin film resonator in this way,it is possible to suppress the occurrence of a standing wave in the areawhere the upper electrode and the lower electrode oppose each other.Thus, the occurrence of a ripple in the communication band can besuppressed.

According to an aspect of the invention, an area where the upperelectrode and the lower electrode oppose each other has a polygonalshape that does not include two parallel sides. By configuring thepiezoelectric thin film resonator in this way, it is possible tosuppress the occurrence of a standing wave in the area where the upperelectrode and the lower electrode oppose each other. Thus, theoccurrence of a ripple in the communication band can be suppressed.

Embodiments 1. Configuration of Piezoelectric Thin Film Resonator

FIG. 1A is a plan view illustrating the piezoelectric thin filmresonator disclosed in Japanese Laid-open Patent Publication No.2007-300430. FIG. 1B is a cross-sectional view of the portion Z-Z inFIG. 1A. In the piezoelectric thin film resonator illustrated in FIGS.1A and 1B, a laminate film including an upper electrode 102, a lowerelectrode 103 and a piezoelectric film 104 is formed on a substrate 101.A resonance portion R101 is an area where the upper electrode 102, thelower electrode 103 and the piezoelectric film 104 overlap with eachother. An air gap 105 is formed in the substrate 101 at a portion underthe resonance portion R101. The shape of the resonance portion R101 issquare.

In the piezoelectric thin film resonator illustrated in FIGS. 1A and 1B,when the magnitude relationship in acoustic velocity between theresonance portion and the non-resonance portion is a relationship thatcauses the lateral leakage of acoustic waves, the acoustic waves leakfrom the resonance portion R101 in the lateral direction towards thenon-resonance portion R102 through the piezoelectric film 104 (see thearrows in FIG. 1A). For this reason, the piezoelectric film 104 in thenon-resonance portion R102 is subjected to patterning such that at leasta portion (end portion 104 a) of the periphery of the piezoelectric film104 is placed inwardly than the periphery of the upper electrode 102.Consequently, the end portion 102 a of the upper electrode 102 becomeslike a hood with respect to the piezoelectric film 104. The upperelectrode 102 having the hood-like end portion 102 a absorbs vibrationsof the piezoelectric film 104 and traps the acoustic waves that leakfrom the lower electrode 103 in the lateral direction.

However, in order to increase the Q-value of the piezoelectric thin filmresonator illustrated in FIG. 1, the over etching amount of thepiezoelectric film 104 needs to be increased to further increase thelength of the hood-like end portion 102 a. When the length of thehood-like end portion 102 a is increased, the mechanical strength of theend portion 102 a is difficult to maintain. For this reason, it has beendifficult to increase the Q-value while maintaining the mechanicalstrength of the end portion 102 a.

FIG. 2A is a plan view illustrating a piezoelectric thin film resonatoraccording to one embodiment. FIG. 2B is a cross-sectional view of theportion Z-Z in FIG. 2A. The piezoelectric thin film resonator includes asubstrate 1, an upper electrode 2, a lower electrode 3 and apiezoelectric film 4. The lower electrode 3 is provided on the surfaceof the substrate 1. The piezoelectric film 4 is provided on top of thesubstrate 1 and the lower electrode 3. The upper electrode 2 is providedon top of the piezoelectric film 4. The piezoelectric film 4 isinterposed between the upper electrode 2 and the lower electrode 3. Atleast a portion of the upper electrode 2 and that of the lower electrode3 oppose each other. A resonance portion R1 is an area where the upperelectrode 2 and the lower electrode 3 oppose each other. As illustratedin FIG. 2A, the resonance portion R1 has a square plane shape. An airgap 5 is formed in the substrate 5 at a portion under the resonanceportion R1.

At least a portion (end portion 2 a in the configuration illustrated inFIGS. 2A and 2B) of the periphery of the upper electrode 2 has areversely tapered cross-sectional shape. An angle Θ1 of the end portion2 a is preferably 90° or more. A lower end 2 b is an end portion of theend portion 2 a in the thickness direction of the upper electrode 2. Thelower end 2 b is placed so as to coincide with or to be in the vicinityof an end portion 4 a of the piezoelectric film 4. It is to be notedthat “the periphery of the upper electrode 2” refers to the portionsurrounded by a thick line in FIG. 2A, and corresponds to the peripherywhen viewed from the normal direction of the main surface of the upperelectrode 2.

In the piezoelectric thin film resonator illustrated in FIGS. 2A and 2B,the upper electrode 2 absorbs vibrations of the piezoelectric film 4.The piezoelectric thin film resonator can trap an acoustic wave W1 and areflected wave W2 reflected by the end portion 4 a of the piezoelectricfilm 4 within the resonance portion R1. Therefore, in the piezoelectricthin film resonator, the leakage of acoustic waves (lateral leakage)from the resonance portion R1 to the lower electrode 3 can besignificantly reduced and the Q-value can be increased.

Furthermore, by setting the angle Θ1 of the end portion 2 a of the upperelectrode 2 to 90° or more and placing the lower end 2 b so as tocoincide with or to be in the vicinity of the end portion 4 a of thepiezoelectric film 4, having a high mechanical strength becomes lessnecessary for the end portion 2 a sticking out like a hood. Thus, it ispossible to ensure that the end portion 2 a sticks out in a largeamount. Accordingly, the Q-value can be further increased.

Here, the end portion 4 a of the piezoelectric film 4 reflects most ofthe acoustic wave W1. Moreover, since leaking in the lateral directionbecomes difficult for the acoustic wave (reflected wave W2) reflected bythe end portion 4 a, the acoustic wave will be trapped in the resonanceportion R1. When the reflected wave W2 is present in the resonanceportion R1 as a lateral standing wave, a ripple may occur in the passband. In order to suppress the occurrence of such a ripple, the shape ofthe resonance portion may be changed to a shape other than square.

FIG. 3A is a plan view illustrating a piezoelectric thin film resonatorwith an elliptic resonance portion R11. FIG. 3B is a cross-sectionalview of the portion Z-Z in FIG. 3A. As illustrated in FIG. 3A, bysetting the shape of the resonance portion R11 to an elliptic shape, itbecomes difficult for conditions for lateral resonance to be met. Thus,it is possible to suppress the occurrence of a lateral standing wavewithin the resonance portion R11.

FIG. 4A is a plan view illustrating a piezoelectric thin film resonatorwith a polygonal resonance portion R21. FIG. 4B is a cross-sectionalview of the portion Z-Z in FIG. 4A. The piezoelectric thin filmresonator illustrated in FIGS. 4A and 4B can suppress the occurrence ofripple. In the piezoelectric thin film resonator illustrated in FIGS. 4Aand 4B, the resonance portion R21 has a polygonal shape that does notinclude two parallel sides. The resonance portion R21 preferably has apentagonal shape. It is to be noted that the shape of the resonanceportion R21 illustrated in FIGS. 4A and 4B is an example. So long as theshape is a polygonal shape that does not include two parallel sides, itmay be a triangular or heptagonal shape. By configuring the resonanceportion in this way, it becomes difficult for conditions for lateralresonance to be met. Thus, it is possible to suppress the occurrence ofa lateral standing wave within the resonance portion R21.

It is preferable that the piezoelectric film 4 is formed using aluminumnitride (AlN). By forming the piezoelectric film 4 using AlN, apiezoelectric thin film resonator having a favorable Q-value can beachieved.

It is preferable that the upper electrode 2 and the lower electrode 3are formed using a material having a high acoustic impedance. In thepresent embodiment, the electrodes are formed using Ruthenium (Ru) as anexample.

As illustrated in FIG. 2A, it is preferable that the area of the air gap5 is larger than that of the resonance portion R1. By configuring inthis way, the resonance portion itself can vibrate freely, resulting inan improvement of the Q-value.

For the electrode films in the piezoelectric thin film resonator,aluminum (Al), copper (Cu), molybdenum (Mo), tungsten (W), tantalum(Ta), platinum (Pt), Ruthenium (Ru), Rhodium (Rh), Iridium (Ir) or thelike may be used. Further, aluminum nitride (AlN), zinc oxide (ZnO),lead zirconate titanate (PZT), lead titanate (PbTiO₃) or the like may beused for the piezoelectric film. Further, silicon (Si), glass or thelike may be used for the substrate.

2. Method of Producing Piezoelectric Thin Film Resonator Example 1

Hereinafter, a production method in which the upper electrode 2 isformed using a SiO₂ film will be described.

As illustrated in FIG. 5A, first, a first sacrificial layer 11 is formedon the substrate 1. Silicon (Si) may be used for the material of thesubstrate 1. Magnesium oxide (MgO) may be used for the material of thefirst sacrificial layer 11. Spattering or vapor deposition may be usedfor forming the layer. In addition to the Si substrate, a quartzsubstrate, a glass substrate, a gallium arsenide (GaAs) substrate, etc.may be used for the substrate 1. The substrate 1 is preferably made of amaterial resistant to etching during the air gap formation process,which will be described below. For the first sacrificial layer 11, it ispreferable to use a material that can be easily etched by an etchantsuch as zinc oxide (ZnO), germanium (Ge) or titanium (Ti).

Next, as illustrated in FIG. 5B, the first sacrificial layer 11 ispatterned in an arbitrary shape. Exposure techniques and etchingtechniques may be used for the patterning.

Then, as illustrated in FIG. 5C, the lower electrode 3 is formed on thesubstrate 1 and the first sacrificial layer 11. Ruthenium (Ru)/chrome(Cr) may be used for the lower electrode 3. Spattering, etc. may be usedfor forming the film.

After that, as illustrated in FIG. 5D, the first sacrificial layer 11and the lower electrode 3 are patterned in an arbitrary shape. Exposuretechniques and etching techniques may be used for the patterning. Atthis time, a path for introducing an etchant for the sacrificial layer(not illustrated) may be formed on the lower electrode 3 and an etchantintroduction hole (not illustrated) for etching the sacrificial layer atthe time of forming the air gap may be formed at the tip of theintroduction path.

Thereafter, as illustrated in FIG. 5E, the piezoelectric film 4 isformed on the substrate 1, the lower electrode 3 and the firstsacrificial layer 11. AlN may be used for the material of thepiezoelectric film 4. Spattering, etc. may be used for forming the film.

Next, as illustrated in FIG. 5F a second sacrificial layer 12 is formedon the piezoelectric layer 4. A silicon oxide (SiO₂) film may be usedfor the material of the second sacrificial layer 12. Spattering, etc.may be used for forming the film. It is preferable to form the secondsacrificial layer 12 so as to have a larger thickness than the upperelectrode 2. The material of the second sacrificial layer 12 is notlimited to SiO₂, and a material having etching selectivity on the upperelectrode 2 is preferably adopted.

Then, as illustrated in FIG. 5G, the second sacrificial layer 12 ispattered. Exposure techniques and etching techniques may be used for thepatterning method. At this time, dry etching may be used for etching theSiO₂ film as the second sacrificial layer 12. Further, the secondsacrificial layer 12 is etched such that the angle of its end portionbecomes an angle Θ2. The angle Θ2 is preferably 90° or more.

Thereafter, as illustrated in FIG. 5H, the upper electrode 2 is formedon the piezoelectric film 4 and the second sacrificial layer 12. Ru maybe used for the material of the upper electrode 2. Spattering, etc. maybe used for forming the film.

Next, as illustrated in FIG. 5I, the SiO₂ film as the second sacrificiallayer 12 is removed by wet etching. As a result, the upper electrode 2can be formed so as to have at least a portion opposing the lowerelectrode 3 and the end portion 2 a of the upper electrode 2 can beformed in a reversely tapered shape having a desired angle Θ1.

Then, as illustrated in FIG. 5J, the piezoelectric film 4 is patternedin a desired shape. Exposure techniques and etching techniques may beused for the patterning. Wet etching may be used for etching the AlN asthe piezoelectric film 4. The piezoelectric film 4 is patterned suchthat the lower end 2 b of the end portion 2 a of the upper electrode 2is placed to coincide with or to be in the vicinity of the end portion 4a of the piezoelectric film 4. Here, dry etching may used for etchingthe AlN.

Thereafter, as illustrated in FIG. 5K, a bump pad 13 is formed on theupper electrode 2 and the lower electrode 3. A lift off method may beused for forming the bump pad 13.

Finally, as illustrated in FIG. 5L, the first sacrificial layer 11 isremoved. An etchant is injected into the introduction hole (notillustrated) for the removal of the layer. The etchant injected into theintroduction hole passes through the introduction path, flows into thebottom of the lower electrode 3 and removes the first sacrificial layer11 by etching the layer. Consequently, an air gap 14 having adome-shaped dilation is formed under the portion where the upperelectrode 2 and the lower electrode 3 oppose each other. The air gap 14bulged like a dome is not illustrated in the drawing.

It is to be noted that the etchant for etching the first sacrificiallayer 11 preferably has less effect on parts of the piezoelectric thinfilm resonator except the sacrificial layer. In particular, the etchantpreferably has less effect on the electrode material on the firstsacrificial layer 11 that comes into contact with the etchant.

Further, by forming the laminate (combined film) composed of the lowerelectrode 3, the piezoelectric film 4 and the upper electrode 2 undersuch conditions that the stress of the laminate becomes compressivestress, the combined film bulges by the time the etching of the firstsacrificial layer 11 ends. Consequently, the dome-shaped air gap 14 canbe formed between the lower electrode 3 and the substrate 1.

The materials of the substrate 1, the upper electrode 2, the lowerelectrode 3 and the piezoelectric film 4 are not limited to thosedescribed above and other materials such as those described in the priorart may be used. Further, instead of the physical air gap 14 describedabove, an acoustic reflection film having a thickness of λ/4 (λ:wavelength of acoustic wave) formed by alternately laminating filmshaving a high acoustic impedance and films having a low acousticimpedance may be used.

Example 2

Hereinafter, a production method in which the upper electrode 2 isformed using a photoresist will be described.

As illustrated in FIG. 6A, first, the first sacrificial layer 11 isformed on the substrate 1. Silicon (Si) may be used for the material ofthe substrate 1. Magnesium oxide (MgO) may be used for the material ofthe first sacrificial layer 11. Spattering or vapor deposition may beused for forming the layer. In addition to the Si substrate, a quartzsubstrate, a glass substrate, a gallium arsenide (GaAs) substrate, etc.may be used for the substrate 1. The substrate 1 is preferably made of amaterial resistant to etching during the air gap formation process,which will be described below. For the first sacrificial layer 11, it ispreferable to use a material that can be easily etched by an etchant,such as zinc oxide (ZnO), germanium (Ge) or titanium (Ti).

Next, as illustrated in FIG. 6B, the first sacrificial layer 11 ispatterned in an arbitrary shape. Exposure techniques and etchingtechniques may be used for the patterning.

Then, as illustrated in FIG. 6C, the lower electrode 3 is formed on thesubstrate 1 and the first sacrificial layer 11. Ruthenium (Ru)/chrome(Cr) may be used for the lower electrode 3. Spattering, etc. may be usedfor forming the film.

After that, as illustrated in FIG. 6D, the first sacrificial layer 11and the lower electrode 3 are patterned in an arbitrary shape. Exposuretechniques and etching techniques may be used for the patterning. Atthis time, a path for introducing an etchant for the sacrificial layer(not illustrated) may be formed on the lower electrode 3 and an etchantintroduction hole (not illustrated) for etching the sacrificial layer atthe time of forming the air gap may be formed at the tip of theintroduction path.

Thereafter, as illustrated in FIG. 6E, the piezoelectric film 4 isformed on the substrate 1, the lower electrode 3 and the firstsacrificial layer 11. AlN may be used for the material of thepiezoelectric film 4. Spattering, etc. may be used for forming the film.

Next, as illustrated in FIG. 6F a photoresist 15 (a second sacrificiallayer in Example 2) is applied to the piezoelectric film 4. Then, thephotoresist 15 is patterned using exposure techniques. At this time, thephotoresist 15 is patterned such that the angle of the end portion ofthe photoresist 15 becomes an angle Θ11. The angle Θ11 is preferably 90°or more.

Thereafter, as illustrated in FIG. 6G, the upper electrode 2 is formedon the piezoelectric film 4 and the photoresist 15. Ru may be used forthe material of the upper electrode 2. Spattering, etc. may be used forforming the film.

Next, as illustrated in FIG. 6H, the photoresist 15 as the secondsacrificial layer is removed using an organic solvent or the like. As aresult, the upper electrode 2 can be formed so as to have at least aportion opposing the lower electrode 3 and the end portion 2 a of theupper electrode 2 can be formed in a reversely tapered shape having adesired angle Θ12. The angle Θ12 is preferably 90° or more.

Then, as illustrated in FIG. 6I, the piezoelectric film 4 is patternedin a desired shape. Exposure techniques and etching techniques may beused for the patterning. At this time, wet etching is preferably usedfor etching the AlN as the piezoelectric film 4. The piezoelectric film4 is patterned such that the lower end 2 b of the end portion 2 a of theupper electrode 2 is placed to coincide with or to be in the vicinity ofthe end portion 4 a of the piezoelectric film 4. Here, dry etching mayused for etching the piezoelectric film 4 (AlN).

Thereafter, as illustrated in FIG. 6J, a bump pad 13 is formed on theupper electrode 2 and the lower electrode 3. Ala off method may be usedfor forming the bump pad 13.

Finally, as illustrated in FIG. 6K, the first sacrificial layer 11 isremoved. An etchant is injected into the introduction hole (notillustrated) for the removal of the layer. The etchant injected into theintroduction hole passes through the introduction path, flows into thebottom of the lower electrode 3 and removes the first sacrificial layer11 by etching the layer. Consequently, the air gap 14 bulged like a domeis formed under the portion where the upper electrode 2 and the lowerelectrode 3 oppose each other. The illustration of the air gap 14 bulgedlike a dome is omitted in the drawing.

Although the air gap 14 is formed between the lower electrode 3 and thesubstrate 1 in Examples 1 and 2, the air gap may be formed in thesubstrate 1.

FIG. 7 is a cross-sectional view illustrating a piezoelectric thin filmresonator in which an air gap is formed in the substrate 1. In thepiezoelectric thin film resonator illustrated in FIG. 7, after formingthe bump pad 13 described in Examples 1 and 2, a resist pattern havingan opening that includes the area where the upper electrode 2 and thelower electrode 3 oppose each other is formed on the backside of thesubstrate 1. Next, dry etching is carried out on the resist pattern suchthat the shape of the side wall becomes substantially perpendicular tothe surface (the surface on which the lower electrode 3, etc. areformed) of the substrate 1. Specifically, the dry etching is carried outby repeating etching using an SF6 (sulfur hexafluoride) gas and a sidewall protection film formation process using a C4F8(perfluorocyclobutane) gas alternately from the backside of thesubstrate 1. As a result, an air gap 5 can be formed at a portion underthe area where the upper electrode 2 and the lower electrode 3 opposeeach other.

FIG. 8A is a diagram illustrating a model of a piezoelectric thin filmresonator used to simulate the relationship between the angle of the endportion of the upper electrode and a Q-value. FIG. 8B is acharacteristic diagram illustrating the relationship between an angle αof the end portion of the upper electrode and a Q-value at ananti-resonance frequency (hereinafter referred to as an anti-resonanceQ). In the characteristic diagram of FIG. 8B, the angle α of the endportion 2 a of the upper electrode 2 is changed using general FEM(Finite Element Method) software and the results of calculating theanti-resonance Q of the piezoelectric thin film resonator are plotted.In the model illustrated in FIG. 8A, the upper electrode 2 and the lowerelectrode 3 are made of Ru and the piezoelectric film 4 is made of AlNand the lower end 2 b of the end portion 2 a of the upper electrode 2 isplaced so as to coincide with or to be in the vicinity of the endportion 4 a of the piezoelectric film 4. As can be seen from the graph,the anti-resonance Q is relatively small when the angle α is 90° or lessbut when the angle becomes 90° or more, the anti-resonance Q increasessharply. Thus, by forming the angle α of the end portion 2 a of theupper electrode 2 in a reversely tapered shape (larger than 90°), theoccurrence of the lateral leakage in the piezoelectric thin filmresonator can be suppressed and the Q-value can be increased. Further,when this piezoelectric thin film resonator having a large Q-value isapplied to a filter or a branching filter, a low-loss filter can beachieved.

Further, by using a photosensitive resin for the material of the firstsacrificial layer 11, a desired pattern can be formed on the firstsacrificial layer 11 only by exposure techniques for the sacrificiallayer. Therefore, without carrying out an etching process, the upperelectrode 2 can be formed on the first sacrificial layer 11 on which apattern is formed.

Further, when removing the first sacrificial layer 11, since the firstsacrificial layer 11 can be easily removed by an organic solvent or thelike, there is no need to consider about the etching selectivity on theupper electrode 2. Thus, the range of choices for the material used forthe upper electrode 2 can be broadened.

3. Configuration of Filter

FIG. 9 is a plan view illustrating a filter using the piezoelectric thinfilm resonators according to the embodiment. The filter illustrated FIG.9 is a ladder filter in which series resonators S1 to S4 and parallelresonators P1 to P3 are connected to each other in a ladder shape. Theseries resonators S1 to S4 and the parallel resonators P1 to P3 have thesame structure as that of the piezoelectric thin film resonatoraccording to the embodiment. The filter includes a substrate 21, upperelectrodes 22 a to 22 c, lower electrodes 23 a to 23 e, piezoelectricfilms 24 a to 24 c and bump pad portions 25 a to 25 e. In FIG. 9, theareas that are hatched with dots are resonance portions. The resonanceportions are each included in the series resonators S1 to S4 and theparallel resonators P1 to P3. The resonance portions in the seriesresonators S1 to S4 and the parallel resonators P1 to P3 have anelliptic shape.

By incorporating the piezoelectric thin film filters according toEmbodiment 1 in a filter, a low-loss filter can be achieved.

Although the configuration of the ladder filter including thepiezoelectric thin film resonators has been described in the presentembodiment, the piezoelectric thin film resonator can be applied toother types of filters such as a lattice filter.

A low-loss filter can be achieved as long as the piezoelectric thin filmresonator according to the present embodiment is used for at least oneof the series resonators S1 to S4 and the parallel resonators P1 to P3in the filter illustrated in FIG. 9.

4. Configuration of Communication Module

FIG. 10 is a diagram illustrating an example of a communication moduleincluding the piezoelectric thin film resonator or the filter accordingto the present embodiment. As illustrated in FIG. 10, a duplexer 62includes a reception filter 62 a and a transmission filter 62 b.Further, reception terminals 63 a and 63 b supporting balanced outputare connected to the reception filter 62 a, for example. Further, thetransmission filter 62 b is connected to a transmission terminal 65through a power amplifier 64. Here, the duplexer 62 can be achieved byusing a duplexer including the piezoelectric thin film resonator or thefilter according to the present embodiment.

At the time of reception operation, the reception filter 62 a allows,among reception signals inputted through an antenna terminal 61, signalsin a predetermined frequency band to pass through and outputs thesignals to the outside from the reception terminals 63 a and 63 b.Further, at the time of transmission operation, the transmission filter62 b allows, among transmission signals inputted through thetransmission terminal 65 and amplified by the power amplifier 64,signals in a predetermined frequency band to pass through and outputsthe signals to the outside from the antenna terminal 61.

By incorporating the piezoelectric resonator or the filter according tothe present embodiment or a duplexer including the resonator or thefilter in a communication module, a low-loss communication module can beachieved.

It is to be noted that the configuration of the communication moduleillustrated in FIG. 10 is an example. Thus, the same effects can beobtained by incorporating the electronic components of the presentinvention in communication modules in other forms.

5. Configuration of Communication Device

FIG. 11 is a diagram illustrating an RF block of a mobile phone terminalas an example of a communication device including the piezoelectric thinfilm resonator, the filter, the duplexer or the above-describedcommunication module according to the present embodiment. Theconfiguration illustrated in FIG. 11 is that of a mobile phone terminalcompatible with the GSM (Global System for Mobile Communications)communication system and the W-CDMA (Wideband Code Division MultipleAccess) communication system. The GSM communication system in thepresent embodiment supports a 850 MHz band, a 950 MHz band, a 1.8 GHzband and a 1.9 GHz band. Although the mobile phone terminal includes amicrophone, a speaker, an LCD display, etc. in addition to thecomponents illustrated in FIG. 11, they are not illustrated in thedrawing because they are not necessary for describing the presentembodiment. Here, a duplexer 73 can be achieved by using a duplexerincluding the piezoelectric thin film resonator according to the presentembodiment.

First, depending on the communication system (W-CDMA or GSM) ofreception signals inputted through an antenna 71, an antenna switchingcircuit 72 selects an LSI required for the operation. When the inputtedreception signals correspond to the W-CDMA communication system, theantenna switching circuit 72 switches the LSI to output the receptionsignals to the duplexer 73. The reception filter 73 a restricts thereception signals inputted to the duplexer 73 to those in apredetermined frequency band by the reception filter 73 a, and thebalanced reception signals are outputted to an LNA 74. The LNA 74amplifies the inputted reception signals and outputs them to an LSI 76.The LSI 176 demodulates the inputted reception signals to voice signalsor controls the operation of each portion in the mobile phone terminal.

On the other hand, at the time of transmitting signals, the LSI 76generates transmission signals. The generated transmission signals areamplified by a power amplifier 75 and are inputted to the transmissionfilter 73 b. Among the inputted transmission signals, the transmissionfilter 73 b only allows signals in a preset frequency band to passthrough. The transmission signals outputted from the transmission filter73 b are outputted to the outside from the antenna 71 through theantenna switching circuit 72.

When the inputted reception signals correspond to the GSM communicationsystem, the antenna switching circuit 72 selects one of the receptionfilters 77 to 80 in accordance with the frequency band of the signalsand outputs the reception signals. The reception signals subjected tothe band restriction by one of the reception filters 77 to 80 areinputted to an LSI 83. The LSI 83 demodulates the inputted receptionsignals to voice signals or controls the operation of each portion inthe mobile phone terminal. On the other hand, at the time oftransmitting signals, the LSI 83 generates transmission signals. Thegenerated transmission signals are amplified by a power amplifier 81 or82 and are outputted to the outside from the antenna 71 through theantenna switching circuit 72.

By incorporating the piezoelectric thin film resonator, the filter, theduplexer or the communication module according to the present embodimentin a communication device, a low-loss communication device can beachieved.

6. Effects of Embodiment, etc

In the piezoelectric thin film resonator according to the presentembodiment, the end portion 2 a of the upper electrode 2 sticks out fromthe end portion 4 of the piezoelectric film 4 and has a hood-like shape.Further, the lower end 2 b of the end portion 2 a is placed so as tocoincide with or to be in the vicinity of the end portion 4 a of thepiezoelectric film 4. Further, the angle between the end portion 2 a andthe upper surface of the upper electrode 2 is set to 90° or more.Consequently, it is possible to trap acoustic waves that propagatethrough the piezoelectric film 4, and thereby not only that losses canbe reduced but also the Q-value can be increased. Further, since it ispossible to increase the length of the hood-like end portion 2 a, theQ-value can be further increased.

In the piezoelectric thin film resonator according to the presentembodiment, it is not necessary to over etch the piezoelectric film 4.Thus, etching time is short and adverse impacts on other materials thatform the piezoelectric thin film resonator are small. In the resonatorillustrated in FIG. 1, the piezoelectric film 104 needs to be overetched to form the hood-like end portion 102 a. Since the exposedportion of the piezoelectric film 104 that is in contact with the upperelectrode 102 is small, the etching rate declines significantly andconsiderable amount of time is required for the over etching. Further,an extension of etching time is highly likely to result in adverseimpacts on other materials that form the piezoelectric thin filmresonator. In the piezoelectric thin film resonator according to thepresent embodiment, however, the piezoelectric film 4 does not need tobe over etched. Thus, the problems as described above can be avoided.

It is to be noted that the substrate 1 in the embodiment is an exampleof the substrate of the present invention. The upper electrode 2 in theembodiment is an example of the upper electrode of the presentinvention. The lower electrode 3 in the embodiment is an example of thelower electrode of the present invention. The piezoelectric film 4 inthe embodiment is an example of the piezoelectric film of the presentinvention. The end portion 2 a in the embodiment is an example of theperiphery of the upper electrode of the present invention.

The disclosure of the present application is useful for film acousticbulk resonators (FBAR) and filters and duplexers using the resonatorsused for mobile communications and high frequency radio communications,such as mobile phones, PHS and wireless LAN.

All examples and conditional language recited herein are intended forpedagogical purposes to aid the reader in understanding the principlesof the invention and the concepts contributed by the inventor tofurthering the art, and are to be construed as being without limitationto such specifically recited examples and conditions, nor does theorganization of such examples in the specification relate to a showingof the superiority and inferiority of the invention. Although theembodiment(s) of the present invention(s) has (have) been described indetail, it should be understood that the various changes, substitutions,and alterations could be made hereto without departing from the spiritand scope of the invention.

The invention claimed is:
 1. A piezoelectric thin film resonator comprising: a substrate; a lower electrode provided on the substrate; a piezoelectric film provided on the lower electrode; and an upper electrode provided on the piezoelectric film, wherein, at least a portion of the upper electrode and that of the lower electrode oppose each other through the piezoelectric film, and at least a portion of a periphery of the upper electrode sticks out from an upper end portion of the piezoelectric film and has a hood-like shape, and at least the portion of the periphery of the upper electrode sticking out like a hood has a reversely tapered shape extending from an edge, or a location adjacent to the edge, of the piezoelectric film.
 2. The piezoelectric thin film resonator according to claim 1, wherein the periphery of the upper electrode is placed so as to coincide with or to be in the vicinity of at least a portion of a periphery of the piezoelectric film.
 3. The piezoelectric thin film resonator according to claim 1, wherein an area where the upper electrode and the lower electrode oppose each other has an elliptic shape.
 4. The piezoelectric thin film resonator according to claim 1, wherein an area where the upper electrode and the lower electrode oppose each other has a polygonal shape that does not include two parallel sides.
 5. A filter comprising a piezoelectric thin film resonator, wherein the piezoelectric thin film resonator includes: a substrate; a lower electrode provided on the substrate; a piezoelectric film provided on the lower electrode; and an upper electrode provided on the piezoelectric film, wherein, at least a portion of the upper electrode and that of the lower electrode oppose each other through the piezoelectric film, and at least a portion of a periphery of the upper electrode sticks out from an upper end portion of the piezoelectric film and has a hood-like shape, and at least the portion of the periphery of the upper electrode sticking out like a hood has a reversely tapered shape extending from an edge, or a location adjacent to the edge, of the piezoelectric film.
 6. The filter according to claim 5, wherein the periphery of the upper electrode is placed so as to coincide with or to be in the vicinity of at least a portion of a periphery of the piezoelectric film.
 7. The filter according to claim 5, wherein an area where the upper electrode and the lower electrode oppose each other has an elliptic shape.
 8. The filter according to claim 5, wherein an area where the upper electrode and the lower electrode oppose each other has a polygonal shape that does not include two parallel sides.
 9. A communication module comprising a piezoelectric thin film resonator, wherein the piezoelectric thin film resonator includes: a substrate; a lower electrode provided on the substrate; a piezoelectric film provided on the lower electrode; and an upper electrode provided on the piezoelectric film, wherein, at least a portion of the upper electrode and that of the lower electrode oppose each other through the piezoelectric film, and at least a portion of a periphery of the upper electrode sticks out from an upper end portion of the piezoelectric film and has a hood-like shape, and at least the portion of the periphery of the upper electrode sticking out like a hood has a reversely tapered shape extending from an edge, or a location adjacent to the edge, of the piezoelectric film.
 10. The communication module according to claim 9, wherein the periphery of the upper electrode is placed so as to coincide with or to be in the vicinity of at least a portion of a periphery of the piezoelectric film.
 11. The communication module according to claim 9, wherein an area where the upper electrode and the lower electrode oppose each other has an elliptic shape.
 12. The communication module according to claim 9, wherein an area where the upper electrode and the lower electrode oppose each other has a polygonal shape that does not include two parallel sides.
 13. A communication device comprising a piezoelectric thin film resonator, wherein the piezoelectric thin film resonator includes: a substrate; a lower electrode provided on the substrate; a piezoelectric film provided on the lower electrode; and an upper electrode provided on the piezoelectric film, wherein, at least a portion of the upper electrode and that of the lower electrode oppose each other through the piezoelectric film, and at least a portion of a periphery of the upper electrode sticks out from an upper end portion of the piezoelectric film and has a hood-like shape, and at least the portion of the periphery of the upper electrode sticking out like a hood has a reversely tapered shape extending from an edge, or a location adjacent to the edge, of the piezoelectric film.
 14. The communication device according to claim 13, wherein the periphery of the upper electrode is placed so as to coincide with or to be in the vicinity of at least a portion of a periphery of the piezoelectric film.
 15. The communication device according to claim 13, wherein an area where the upper electrode and the lower electrode oppose each other has an elliptic shape.
 16. The communication device according to claim 13, wherein an area where the upper electrode and the lower electrode oppose each other has a polygonal shape that does not include two parallel sides. 